If you're son already understands the distinction between a description and an explanation I'd say he's off to an excellent start.

That being said, I'm not a physicist, but my understanding is that no one know exactly what gravity is. We understand it's effects pretty well from both classical physics as well as via the curvature of space. But we don't know what causes it.

You should go back to your son and tell him humbly that we don't know. I would explain to him the curvature of spacetime using a heavy ball in a table cloth as an example. This unravels one layer of the mystery but leaves other unanswered questions. WHY does an object curve spacetime?? Unanswered questions do wonders to inspire curious children like your son.

BTW, Watch this video from Richard Feynman. He tells an amazing story about him asking his father a similar question about "inertia". http://www.youtube.com/watch?v=695Flhmjmg4&feature=relat...